Experimental Comparison of Two Approaches for Solar Wavefront Sensing

The Sun presents unusual problems for wavefront sensing. Unlike the nighttime sky, the Sun does not provide natural, high-contrast point sources, and creation of laser beacons bright enough to be visible against the solar disk poses major technical and operational problems. Small sunspots and pores can provide satisfactory substitutes for point sources, but these are available for only a tiny fraction of the solar disk. Wavefront sensing at arbitrary locations on the Sun requires a sensor capable of using the ubiquitous solar granulation as its target. Solar granulation is extended (its characteristic angular scale is about one arcsecond), unbounded (the angular extent of the composite granulation pattern greatly exceeds the isoplanatic angle), low contrast (a few percent), and both spatially and temporally variable (the typical evolution time scale is minutes). Conventional wavefront sensors such as shearing interferometers and simple Shack-Hartmann position sensors have difficulty dealing with targets having these characteristics, and therefore are not well-suited for general solar imaging.